Amorphous Semiconductors for Gamma Radiation Detection (ASGRAD)
Total Page：16
File Type：pdf, Size：1020Kb
PNNL-22277 PL06-136-PD05 FY08 Annual Report: Amorphous Semiconductors for Gamma Radiation Detection (ASGRAD) B. R. Johnson B. J. Riley J. V. Crum J. V. Ryan S. K. Sundaram J. McCloy A. Rockett, UIUC February 2009 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor Battelle Memorial Institute, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or Battelle Memorial Institute. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. PACIFIC NORTHWEST NATIONAL LABORATORY operated by BATTELLE for the UNITED STATES DEPARTMENT OF ENERGY under Contract DE-ACO5-76RL01830 This work was conducted in part at the Environmental Molecular Sciences Laboratory (EMSL), which is operated by the Office of Biological and Environmental Research of the U.S. Department of Energy (DOE) at Pacific Northwest National Laboratory (PNNL). Printed in the United States of America Available to DOE and DOE contractors from the Office of Scientific and Technical Information, P.O. Box 62, Oak Ridge, TN 37831-0062; ph: (865) 576-8401 fax: (865) 576 5728 email: [email protected] Available to the public from the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Rd., Springfield, VA 22161 ph: (800) 553-6847 fax: (703) 605-6900 email: [email protected] online ordering: http://www.ntis.gov/ordering.htm PNNL-22277 PL06-136-PD05 FY08 Annual Report: Amorphous Semiconductors for Gamma Radiation Detection (ASGRAD) B. R. Johnson B. J. Riley J. V. Crum J. V. Ryan S. K. Sundaram J. McCloy A. Rockett, UIUC February, 2009 Work performed for the Office of Defense Nuclear Nonproliferation (NA-20) Office of Nonproliferation Research and Development (NA-22) under Project Number PL06-136-PD05 Prepared for the U.S. Department of Energy under Contract DE-AC05-76RL01830 Pacific Northwest National Laboratory Richland, Washington 99354 Summary High Z, high resistivity, amorphous semiconductors were designed for use as solid-state detectors at near ambient temperatures; their principles of operation are analogous to single-crystal semiconducting detectors. Compared to single crystals, amorphous semiconductors have the advantages of rapid, cost- effective, bulk-fabrication; near-net-shape fabrication of complicated geometries; compositional flexibility; and greater electronic property control. The main disadvantage is reduced-charge carrier mobility. This project developed amorphous semiconductor materials for gamma detection applications that leverage material advantages while mitigating the limitations of this class of materials. This development effort focused on materials synthesis, characterization of physical properties, characterization of electrical properties, radiation detector design, as well as detector signal collection and processing. During the third year of this project, several important milestones were accomplished. These include: 1) We were able to collect pulse-height spectra with an amorphous semiconductor-based radiation detector. This was done using both an alpha source and a gamma source. 2) We identified an anomalous conduction phenomenon in certain amorphous semiconductors. When cooled to approximately 250K, they displayed enhanced conductivity by several orders of magnitude. This increase is thought to be due to increased charge carrier mobility rather than an increase in charge carrier population. This is significant for radiation detection applications because it indicates there could be a temperature regime where amorphous semiconductors could have significantly enhanced performance. 3) We developed the capability to perform automated, high-precision, electronic property (conductivity, resistivity, Hall mobility) measurements of highly resistive materials as a function of temperature, source voltage, and source current. 4) Three refereed journal papers were published. A fourth paper has been submitted 5) We mentored an undergraduate student and contributed towards their educational and professional development as they assisted in making and testing specimens. 6) The collaborative working relationship was developed with Prof. Angus Rockett at the University of Illinois at Urbana-Champaign (UIUC). They performed key materials characterization experiments to evaluate the electrical performance of different metallization schemes. The development of Schottky barrier contacts for the higher conductivity amorphous semiconductors was identified as one of the most significant goals to achieve in FY 2008. Initial results looked quite promising, but the results were not reproducible. Significant effort and attention was directed towards this effort by both PNNL and UIUC, but a suitable rectifying contact material and a reproducible deposition process has not been found. Additional details about the status of this task are addressed in the body of this report. The project was scoped and budgeted to focus on materials synthesis and characterization. The materials and processing challenges associated with developing high-performance contacts on novel semiconducting materials was beyond the scope of this project. That said, the strategy was to leverage traditional semiconductor technologies as a best-effort approach. The concept was to use lower resistivity materials with better charge carrier mobility properties and operate them under reverse bias to create a low-noise, high-resistivity condition that can be switched to a low-resistivity, high conductivity condition iii under exposure to radiation events, and then off again. The key to creating effective Schottky barrier contacts lies in finding a metal with a work function that is suitably matched to the semiconductor. Professor Rockett’s research group at the University of IL has been using Kelvin probe force microscopy using an AFM to measure and evaluate the work function of various contact metals and amorphous Cd- Ge-As. They were able to make modest progress on this effort, and their results are presented in the appendix to this report. iv Abbreviations and Acronyms AST Arsenic-selenium-telluride; As40Se60-xTex BSE Back-scattered electron CVD Chemical vapor deposition CGA Cadmium-germanium-di-arsenide; CdGexAs2 EDS Energy dispersive spectroscopy EMSL Environmental Molecular Sciences Laboratory IR Infrared IV Current - voltage KPFM Kelvin probe force microscopy NIST National Institute for Standards and Technology NOMSL Non-oxide Materials Synthesis Laboratory PNNL Pacific Northwest National Laboratory PPMS Physical property measurement system SEM Scanning electron microscopy SOW Statement of work UHV Ultra-high vacuum UIUC University of Illinois at Urbana-Champaign UV-VIS-NIR Ultraviolet visible near infrared XRD X-ray diffraction Z Atomic number v Contents 1.0 Introduction ........................................................................................................................................ 1.1 2.0 Materials Synthesis ............................................................................................................................ 2.1 2.1 Synthesis Method ...................................................................................................................... 2.1 2.2 New Chemistries ....................................................................................................................... 2.2 2.3 Progress on Identifying the New Metastable Phase ................................................................. 2.4 2.4 Processing Summary ................................................................................................................ 2.5 3.0 Electrical Property Characterization .................................................................................................. 3.7 3.1 Current-voltage curves .............................................................................................................. 3.7 3.2 Hall mobility ............................................................................................................................. 3.8 4.0 Radiation Response Testing ............................................................................................................... 4.8 4.1 DC Ionization Tests .................................................................................................................. 4.9 4.2 Pulse Testing ........................................................................................................................... 4.12 4.3 Summary ................................................................................................................................. 4.14 5.0 Collaboration with the University of Illinois at Urbana-Champaign (UIUC) .................................. 5.1 5.1 Conclusions of the materials characterization at UIUC ........................................................... 5.1 5.2 Schottky